Don't be inhibited or embarrassed. Keep writing and see where your ideas
take you.

Read everything you can get your hands on, especially books that are
considered unsuitable for someone of your age. Try and read as a writer:
work out how the author is creating certain effects and analyse what works
for you as a reader.

Use notebooks. When you're out and about and you see something striking or
beautiful or weird, describe it. Eavesdrop on conversations. Make up stories
about people you pass on the street.

When you're editing something you've already written, look closely at the
first couple of pages. How much of it is really necessary, and how much was
you warming up and getting your thoughts together?

Find a private space and read your work out loud, especially the dialogue.
If sentences sound awkward, they're probably awkward on the page too.